A New Method of Synthesizing Phenalen-1-one: Reduction of 3-Hydroxyphenalen-1-one Using NaBH₄ and Lanthanoid Chlorides

Abstract

Phenalen-1-one was obtained in considerable yield by reducing 3-hydroxyphenalen-1-one. Most of known preparation methods are not very practical, either because their yields are very poor or because their processes have many steps. This regioselective 1,2-reduction proceeded by the action of NaBH₄ and various cations of rare-earth elements and metals. The yields of phenalen-1-one were examined as a function of typical lanthanoids, molar ratios of lanthanoid ions to 3-acetoxy-phenalen-1-one, and differing methods of protecting the hydroxyl group. Lanthanum chloride (LaCl₃) gave the greatest yield (45.3%) of phenalen-1-one at molar ratios higher than a third, probably because La³⁺ ion is a hard acid and coordinates easily to a hard solvent such as methanol. Further, it has the largest ionic radius among all lanthanoid ions.

Keywords:

• α-Enone
• 3-hydroxyphenalen-1-one
• lanthanoid salt
• NaBH₄
• phenalen-1-one
• 1,2-reduction

ACKNOWLEDGMENTS

The authors thank Prof. Masatugu Kajitani in Sophia University for kindly providing the EI–mass and elementary analyses, Prof. Yoshifumi Akama of Meisei University for discussions about lanthanoid ions, and Miss Michiko Nagao for experimental assistance.

Notes

Note. The term “trace” signifies a yield of <1.0%.

Note. The term “trace” signifies a yield of <1.0%.